hsp90 is a key molecular chaperone protein in the cell. It participates in producing and maintaining the active states of many important regulatory proteins including a number of transcription factors and protein kinases. The activities of steroid receptors are dependent on hsp90 chaperoning and we have used the progesterone receptor (PR) as a model client protein for studies on the mechanism of hsp90 action. The objective of the studies proposed is to define the properties and the mechanism of hsp90 binding to client proteins and to gain information on the consequences of this important interaction. Three specific aims are proposed. In Aim 1, methods will be developed to use in describing directly the binding of hsp90 to PR. This interaction is complex and normally requires additional co-chaperones. However, by mild heating one can induce hsp90 to bind PR without assistance by other proteins allowing the study of several aspects of client binding in a simplified system. In Aim 2, the binding of hsp90 to small peptides will be studied as a second approach to understanding the client binding activity of hsp90. Peptides will be selected from random libraries using phage display methods and their binding sites on hsp90 will be characterized. In Aim 3, this new information on hsp90 will be used to study the roles and the regulation of client binding in the more complex system where hsp90 cooperates with other chaperones and co-chaperones to produce active forms of client proteins. At present, the mechanism by which hsp90 selects and binds client proteins is unknown. These studies should provide valuable new insights into this mechanism which is key to understanding the biological significance of hsp90. [unreadable] [unreadable]